Semiconductor optical amplifiers (SOAs) typically have optical amplification factors that vary under different conditions. For example, the optical amplification factor of a semiconductor optical amplifier has a correlation with temperature. More specifically, the optical amplification factor of a semiconductor optical amplifier increases with a decrease in the temperature of an active layer of the semiconductor optical amplifier and decreases with an increase in the temperature of the active layer. Therefore, the temperature of the semiconductor optical amplifier is controlled using a thermo-electric cooler (TEC).
Furthermore, the optical amplification factor of a semiconductor optical amplifier has a correlation with driving current supplied to an active layer thereof. More specifically, the optical amplification factor of a semiconductor optical amplifier increases with an increase in driving current and decreases with a decrease in driving current. Accordingly, a technology for controlling the optical amplification factor of a semiconductor optical amplifier by using driving current is disclosed (for example, “The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, CPM, electronic parts and materials (CPM 2001-125),” Vol. 101, No. 517 (20011214) pp. 23-28 (The Institute of Electronics, Information and Communication Engineers ISSN: 09135685)).